Hitherto, there has been a demand for a plasma torch which is capable of cutting material, such as steel, stainless steel, etc., with high precision and without adherence of molten metal. (hereinafter referred to as dross), which has a narrow cutting width, which is even capable of cutting thick plates, and which has a long life. With regard to such prior art, one of the present applicants has proposed a transferred plasma torch, for example, in Japanese Utility Model Application No. 1-72919. For example, each of FIGS. 7 and 8 is a cross-sectional view of a nozzle and electrode section of a conventionally proposed transferred plasma torch, wherein swirling air currents are produced in the operating gas. In the transferred plasma torch 50 of FIG. 7, a switch 53 is operated to transfer the arc, formed between a nozzle 52 and an electrode member 51a of an electrode 51, to a workpiece 54 to be cut. In this plasma torch 50, a swirler member 55 is inserted near the electrode 51, disposed within the nozzle 52, and a plurality of holes 55a are obliquely formed downwardly therein. The operating gas, which has passed through the plurality of holes 55a, becomes swirling currents and is successively accelerated in an acceleration section 52a, formed into a V shape with a gentle inclination at the front end of the nozzle 52, and reaches a nozzle restriction section 52b for restricting the arc let 56 such that it moves in a straight line.
In plasma torch 60 of FIG. 8, a swirler member 63 is inserted near an electrode 62, disposed in nozzle 61, and a plurality of holes 63a are formed in the swirler member 63 perpendicular to axial center Z of the plasma torch 60 and tangential with respect to the inner peripheral face of the swirler member 63. At the front end of the nozzle 61 below the electrode 62, there is disposed a velocity reduction space 61a below and apart from the lower end of an electrode member 62a of the electrode 62. The operating gas, which has passed through the plurality of holes 63a, becomes swirling air currents; and in the velocity reduction space 61a, these swirling air currents allow arc jet 56 to be held in a low-pressure space formed in the center axis and therearound. Since the nozzle 61 has the velocity reduction space 61a at the upstream side, it is capable of preventing deflection of the arc jet 56 which is ejected from the nozzle restriction section 61b, so that it is generated with a high degree of straightness, which results in excellent cutting of the workpiece 54.
However, in such above-described conventional transferred plasma torches, when in conventional use a current is made to flow through an electrode and a conventional operating gas flow rate is supplied, it is extremely difficult to achieve cutting of a workpiece in a dross free state. This is thought to be very difficult to achieve even when the conditions are changed.
Another different prior art is known, in which cutting in a dross free state is achieved by a method which comprises cutting a workpiece by an arc jet having the operating oxygen gas further enveloped by an oxygen curtain during cutting (refer, for example, to Japanese Patent Laid-Open No. 59-229282). However, the use of oxygen for the curtain results in increased gas consumption as well as a reduced precision in the dimensions of the cut face or the like due to burning.
The present invention has been achieved to overcome the above-described problems of the prior art, and relates to a plasma torch and, more particularly, to a plasma torch in which a transferred arc jet is generated, wherein dross adhesion does not occur, the arc jet is stable, and the nozzle, etc., has a long life.